Usually, fuel stored in a fuel tank is evaporated and fuel evaporation gas is produced. Generally, the fuel tank is tightly closed, and an internal pressure is increased as the evaporation gas is increased. Therefore, if the pressure is not released outside, there is a possibility that the tank is broken. However, for example, if gasoline fuel is evaporated, since harmful hydrocarbon (HC) component is included, such gas should not be released to the atmosphere in terms of environmental performance. Hence, in a conventional internal combustion engine, there is prepared an evaporation gas purge apparatus in which the evaporation gas is adsorbed by activated carbon in a canister, and the evaporation gas is introduced (purge controlled) into an intake system together with new gas by suction negative pressure of the engine. For example, the following Patent Documents 1 to 4 disclose various modes of purge control of the evaporation gas purge apparatus.
The Patent Document 1 describes purge control in which if fuel is light fuel in which evaporation gas is increased, a purge flow rate of the evaporation gas is reduced, and if fuel is heavy fuel in which evaporation gas is reduced, the evaporation gas is positively purged. The Patent Document 2 describes purge control in which the heavier the fuel is, the more the purge flow rate of evaporation gas is suppressed.
Here, when fuel is alcohol-mixed fuel, an evaporation degree is varied in accordance with alcohol concentration (mixed ratio of alcohol fuel) in the fuel tank. Therefore, an air/fuel ratio in the combustion chamber is deviated from a target air/fuel ratio at the time of purge control, and engine output performance and exhaust emission performance are deteriorated. Therefore, when fuel is alcohol-mixed fuel, it is preferable that a purge flow rate of the evaporation gas is adjusted in accordance with alcohol concentration in the fuel tank as described in the Patent Documents 3 and 4. A technique of the Patent Document 3 is constituted such that purge control is performed based on the alcohol concentration in the fuel tank and the evaporation degree of the alcohol-mixed fuel.
Patent Document 1: Japanese Patent Application Laid-open No. H4-136469
Patent Document 2: Japanese Patent Application Laid-open No. H6-81730
Patent Document 3: Japanese Patent Application Laid-open No. H1-106971
Patent Document 4: Japanese Patent Application Laid-open No. H5-99083